1. Field of the Invention
The invention relates to a thermal transfer film of the generic type comprising a reactive polymer composition which is suitable as donor element for imaging a lithographic printing-plate cylinder, in particular for offset printing, by laser-induced transfer. The invention also relates to a process for the production of the film and to intermediates for this purpose.
2. Description of the Related Art
A printing process is known in which a printing-plate cylinder is provided with plastic in a punctiform and imagewise manner. For an offset process, this printing-plate cylinder is then coated with printing inks, and the printing ink in the ink-carrying areas is taken up by a rubber roll and transferred onto the substrate to be printed. For rapid change of the print motifs, in particular for small runs, it is desired to carry out the operation within an apparatus firstly wherever possible with computer control and secondly without changing movable parts. The printing apparatus presented in U.S. Pat. No. 5,601,022 meets this demand.
The printing-plate cylinder used in the above-mentioned apparatus is covered in a punctiform and imagewise manner with a polymer which originates from a thermal transfer ribbon. In order to obtain a lithographic printing plate which is suitable for offset printingxe2x80x94this means clear separation of the hydrophilic areas (the parts on the printing-plate cylinder which are not covered with polymer) and hydrophobic areas (the parts on the printing-plate cylinder which are covered with polymer and represent the ink-carrying areas later during the printing operation) on the printing-plate cylinderxe2x80x94certain physical and chemical parameters of the thermal transfer film, in particular for a thermal transfer ribbon, must be established and optimized.
The object of the present invention is to provide an improved thermal transfer film for the imaging of a printing-plate cylinder by laser-induced transfer of a polymer from a donor layer of the thermal transfer film, where the following requirements are to be observed:
1. the support must meet the mechanical requirements of transport, the optical requirements of transmission of a laser beam and the thermal requirements during warming of the applied coating;
2. the coating which is located on the film and is to be transferred in a punctiform and imagewise manner should adhere strongly to the printing-plate cylinder and offer a service life which is adequate for the longest possible run with constant print quality; and
3. it should be possible for the applied polymer on the printing-plate cylinder to be removed in a simple and environmentally friendly manner and rapidly after the printing operation in order that a new printing operation can begin as soon as possible.
4. The polymer composition applied in pixel form should have an ideal cylindrical shape or approximate to this shape.
The above object and further objects revealed in the description below have been achieved by a thermal transfer film including a substrate layer with a donor layer applied thereto. The substrate layer consists of at least one polymer composition which is mechanically stable at a temperature greater than 150xc2x0 C., and has a transmissivity greater than 70% for light having a wavelength of 700 to 1600 nm. The donor layer includes a substance which is able to convert the radiation energy of the incident light into heat energy, a polymer which contains acidic groups and/or substituted amide groups, and optionally a wetting aid.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.